Conventionally, normally-off type and normally-on type field effect transistors are known.
In the normally-off type field effect transistor, when a threshold voltage is 0 V or positive while a gate voltage is 0 V a drain current does not flow (in an off state). Such a normally-off type field effect transistor is suitable for protecting external circuits before turning on a power source or upon power loss.
Meanwhile, in the normally-on type field effective transistor, when a threshold voltage is negative while a gate voltage is 0 V, a drain current flows (in an on state). The normally-on type field effect transistor has good characteristics, such as a high breakdown voltage or a low on-resistance, compared to the normally-off type field effect transistor.
For this reason, there is a known field effect transistor that includes a combination of a normally-off type E (enhancement)-mode field effect transistor and a normally-on type D (depletion)-mode field effect transistor (see Patent Documents 1, 2, 3, and 4). Such a combined field effect transistor is of the normally-off type as a whole, but achieves high breakdown voltage and low on-resistance.
Referring to, for example, FIG. 3A and a third column, lines 5-10 in Patent Document 1, a field effect transistor is known to include a combination of a field effect transistor with Vth,2<0 V (whose threshold voltage is negative and which is of the normally-on type) and another field effect transistor with |Vth,2|<Vbk,1 (whose threshold voltage Vbk,1 is positive and which is of the normally-off type).
Referring to, for example, FIG. 1 and paragraphs [0020] and [0022] in Patent Document 2, a second recess 4 is of the normally-on type, while a first recess 8 is of the normally-off type.
Regarding the second recess 4, it is desirable to see the paragraph [0022] in Patent Document 2, specifically, “the depth of a second recess is adjusted such that a two-dimensional electron gas layer is formed with a gate 5 floating” and “even when an off-voltage is applied to the gate 5, a two-dimensional electron gas layer is formed if a high voltage is not applied to a drain electrode 18”. In the vicinity of the second recess, the field effect transistor is of the normally-on type because the two-dimensional electron gas layer is formed (i.e., current flows) while the off-voltage is applied to the gate 5, or the gate 5 is floating.
Regarding the first recess 8, it is desirable to see a paragraph [0020] in Patent Document 2, specifically, “In a range C1 corresponding to the first recess 8, a semiconductor layer 16 has no heterojunction surface. Thus, in the range C1, the two-dimensional electron gas layer is not formed in the semiconductor layer 16 due to a difference in the bandgap . . . . When no on-voltage is applied to the gate 5, the voltage of the gate 5 becomes a ground voltage with no accumulation layer formed in a first channel C1, whereby electrons cannot move through the first channel C1”. In the vicinity of the first recess 8 (range C), the field effect transistor is of the normally-off type because electrons cannot move (i.e., current does not flow) while the on-voltage is not applied to the gate 5.
Referring to, for example, FIG. 1 and a paragraph [0065] in Patent Document 3, it shows “In this way, a first gate electrode 10 and a second gate electrode 20 are operated in the normally-on manner, which simplifies the structure of the semiconductor device and makes the manufacture thereof easier . . . . However, the embodiment is not limited thereto, and alternatively, the second gate electrode 20 may be operated in the normally-off manner.” As mentioned above, this patent document discloses that the first gate electrode 10 is operated in the normally-on manner, while the second gate electrode 20 is operated in the normally-off manner.
Note that Patent Document 4 also discloses a combination of the normally-off type field effect transistor and the normally-on type field effect transistor (see, for example, Abstract).
As described above, in the field effect transistor including the combination of the normally-off type field effect transistor and the normally-on type field effect transistor, a drain-to-source voltage applied to the normally-off type field effect transistor becomes approximately equal to a difference between the threshold voltage (0 V or a positive value) of the normally-off type and the threshold voltage (a negative value) of the normally-on type.